The effect of rotation on the thermal instability of stratified galactic atmospheres – II. The formation of high-velocity clouds

Abstract

Whether High Velocity Clouds (HVCs) can form by condensation of the hot (T ∼ 10 6 K) Galactic corona as a consequence of thermal instabilities has been controversial. Here we re-examine this problem and we suggest that rotation of the corona might be a missing key ingredient. We do this by studying the evolution of the models of rotating galactic coronae presented in Sormani et al. (2018) under the presence of cooling and thermal conduction. We combine a linear stability analysis with the results of local and global hydrodynamical simulations. We find that condensations are likely to occur in regions where the corona has substantial rotational support. Under reasonably general assumptions on the rotation profile of the corona, the locations where condensations are expected are in remarkable agreement with the observed location of the major non-magellanic HVCs complexes in our Galaxy (namely, at distances 15 kpc from the Sun and within 30 • from the disc plane). We conclude that HVCs can form by thermal instabilities provided that (i) the corona is rotating substantially in the inner (R 50 kpc) parts, as suggested by current observational data and predicted by cosmological simulations of galaxy formation; (ii) close to the disc the corona is well-represented by a nearly-equilibrium stratified rotating structure (as opposed to a fast cooling flow). Our results also suggest that a better understanding of the disc-halo interface, including supernova feedback, is critical to understand the origin of HVCs.

Predictions for the X-ray circumgalactic medium of edge-on discs and spheroids

“…While modeling hot gaseous haloes under the assumption of spherical hydrostatic equilibrium may be a good assumption for spheroid-hosting haloes, the deviation from sphericity around disc galaxies (Oppenheimer 2018) can be tested by the axial ratios of hot haloes beyond 10 kpc around discs. Denser, co-rotating gas along the semi-major axis can better facilitate the cooling of the hot CGM and the condensation of ∼ 10 4 K phase (Sormani & Sobacchi 2019). Targeting nearby disc galaxy CGMs with eROSITA after the completion of the eRASS:8 survey may be worth the invest-ment to ascribe a comprehensive theoretical explanation to multiwavelength observations of the CGM.…”

“…While X-ray emission probes will likely not be able to observe these velocities in the foreseeable future, the determination via azimuthal emission of denser equatorial gas has important implications for how disc galaxies accrete material from the hot CGM. If we consider the precipitation criterion of 𝑡 cool /𝑡 ff 10 (Sharma et al 2012) for gas to cool, the rotating models of Sormani et al (2018) favor condensation of cool gas near the disc axis by i) lowering the cooling time (𝑡 cool ) with increased density, and ii) raising the effective free-fall time (𝑡 ff ) via rotational support (Sormani & Sobacchi 2019). This later paper argues cooling from the hot CGM within ∼ 30 • of the disc promotes the formation of high-velocity cloud structures.…”

Inflow of low-metallicity cool gas in the halo of the Andromeda galaxy

“…These perturbations could also be caused by external processes like the cosmological accretion of IGM and/or satellite galaxies (Nelson et al 2020;Esmerian et al 2021). However, whether or not these instabilities can develop at all is still under debate (see Binney et al 2009;McCourt et al 2012;Sharma et al 2012;Nipoti & Posti 2013Sormani & Sobacchi 2019).…”

A Kinematic Analysis of Ionised Extraplanar Gas in the Spiral Galaxies NGC 3982 and NGC 4152

“…In this scenario the H𝛼-emitting material would not originate from the circulation of fountain clouds, but it would rather trace gas accretion from the CGM (Kaufmann et al 2006;Zheng et al 2017) produced either by the spontaneous cooling of the hot corona (e.g. Voit et al 2015;Sormani & Sobacchi 2019) or by cold filaments penetrating the hot gas down to the disc (e.g. Fernández et al 2012;Mandelker et al 2019).…”